When a glass of water sits for several hours, why do small bubbles form in the water?

Every morning I fill up a glass with water and put it on my desk. I drink about a 1/4 of it. Then, sometimes it sits there the rest of the morning, like if I’m not working at my desk or I choose to drink coffee instead. After lunch, when I get fresh water, I’ve noticed that there are little bubbles in the water. Right now, the water has been sitting for an hour and there’s no bubbles. But, I know there will be later. What’s going on?